Method of painting electrostatically nonconducting articles



V. W. TOLLE ETAL METHOD OF PAINTING ELECTROSTATICALLY NONCONDUCTINGARTICLES June 5, 1951 Filed March 11, 1947 HEAT/N6 Z emu/vase M 0 as K 5O w; Em v N i M A VP V 6 Patented June 5, 1951 T I UNITED STATES PATENTOFFICE METHOD OF PAINTING ELECTROSTATI- CALLY NONCONDUC' IING ARTICLESVance W. Tolle and Paul E. Ludy, Dayton, Ohio,

assignors to General Motors Corporation, Detroit, Mich., a corporationof Delaware Application March 11, 1947, Serial No. 733,756

heated to a selected and controllin temperature.

In carrying out the above object, it is a further object in some cases,to heat the paint prior to its application to a selected and controlledtemperature.

Another object of the invention is to provide a method for paintingrubber steering wheels and the like in an electrostatic field whereinthe rubber steering wheel includes an insert of metal and wherein theentire steering wheel is heated to a temperature of between 175 and 225F., whereupon the paint is applied in the electrostatic field.

In carrying out the above object, it is a further object, inlsome cases,to heat the paint prior to panying drawings wherein Fig. '1 illustratesschematically a plan view of an apparatus for electrostatically sprayingsteering. wheels and wherein Fig. 2 is a section taken through line 22of Fig. l.

The use of electrostatic fields in the applicafor such spraying is shownand described in" Ransburg et al. Patent No. 2,247,963. The basicprinciple of painting in an electrostatic field is to cause the articleto be sprayed to act as one electrode of the circuit and to provideanother electrode surrounding the article connected to the other side ofthe circuit. Both electrodes are attached to apparatus for producing anelectrostatic charge which causes an electrostatic field to be set upbetween the two electrodes. The article to be sprayed is positivelycharged while the other electrode carries the negative charge. Thepaint, when sprayed in this field, is attracted to the positivelycharged electrode since the positive electrode is always of a greatersize than the negative electrode. In this manner,

paint particles are directed to their point of application whereby lossof paint is reduced and a better application thereof is obtained.

Spraying of articles in electrostatic fields is very successful wherethe articles themselves are 'of a conductive nature for example, metal.We

have found, however, that where non-conductive articles are to besprayed, that such articles must be backed-up with a satisfactory metalelectrode or must include metal inserts therein in order to accomplishthe end desired. In each case, the application of the paint is notentirely satisfactory since the non-conductive surface to be coated doesnot appear to properly attract the paint to cause a uniform layer orcoating to form.

In order to correct this difficulty and make the application ofelectrostatic spraying of nonconductive articles completelysatisfactory, we have found that by heating the article to a relativelyhigh temperature that the paint particles are more uniformly attractedwhereby a satisfactory paint coating is obtained.

Specifically, we heat the non-conductive article, for example, selectedfrom the group consisting of rubber, reclaimed rubber, and syntheticelastomers, such as, butadiene-styrene copolymers or mixtures thereofetc., in the form of a steering wheel etc., having a metal inserttherein, to a temperature between 150 to 250 F., wherein thistemperature is maintained at the surface of the article during theelectrostatic spraying operation. Preferably, a temperature of from 200to 250 F., gives optimum results but in all cases throughout thetemperature range noted, the results are highly superior to thoseobtained when no heat is utilized.

In some cases, for example with butadienestyrene copolymer articles itis desirable to include a conductive material such as a conductivecarbon black, metal powder etc., in relatively small quantities, forexample, from 1% to 2% by weight in the compound. This carbon blacktends to make the compound a better electrode in the electrostaticequipment. The heating of the article should be carried out prior to thespraying of the paint as before. It is understood that the black can beused up to 30% and above but it serves no useful purpose for thepainting over the preferred limits.

Another embodiment relates to the heating of the paint per se. We havefound that if the paint is maintained at a temperature of between andF., that the entire process is further improved and more reliableresults are obtained. This is particularly true in low ambienttemperatures which causes a roughness in the paint surface if the paintremains cold. By heating the paint to a standardized temperature duringthe spraying operation, all of these variable factors are eliminated anda surface of greater uniformity is obtained. Also the quantity ofthinner may be reduced to provide greater thickness of paint film.

Referring particularly to the drawings, a schematic apparatus forspraying steering wheels is shown wherein in Fig. 1, indicates a heatingchamber having a conveyor 22 passing therethrough. The conveyor 22 has aplurality'of spindles 24 thereon which are adapted to support steeringwheels 26 which are to be painted.

so that uniform spraying of the paint may be obtained. In practice, weprefer to turn the wheels about 12 revolutions during the sprayingoperation. The negative electrode is shown at 28 .and consists of aplurality of wires-which are disposed as a canopy over the conveyor sothat the wheels pass thereunder. The spindles and the wheels which actas one electrode and the other electrode are suitably connected to anapparatus 30,:capable of establishing an electrostatic fieldtherebetween. A plurality of spray nozzles 32 are provided which spray.a mist of paint within the electrostatic field whereby the particles ofpaint are attracted to the wheels and uniformly coat the same. One ormore coats of paint may be utilized wherein the wheels are nature aretobe coated, and in this respect, it .is manifest that the scope of theinvention is such to adequatelyfcover these variations.

While the embodiments of the present invention as herein disclosed,constitute preferred forms, it is to be understood that other formsmight he adopted, all coming within'thescope of the claims which follow.

What is claimed islas follows:

1. In the method of coating a substantially electrically non-conductivearticle made from a material selected from'the class consisting ofrubber and synthetic elastomers, which article includes a metal inserttherein, with a solvent containing paint, the steps of; heating thearticle to a temperature of from 150 F. to 250 F.,

passing the heated article into an electrostatic field of definedcharacter, and simultaneously Each spindle preferably rotates on theconveyor q "painted.

4 spraying a solvent containing paint on the article whereby a uniform,strongly adhering coating is obtained.

2. In the method as claimed in claim 1, including the added step ofheating the paint prior to spraying to a temperature of from to 120 F.

3. The method as claimed in claim 1,, wherein the non-conductive articleincludes a conductive material dispersed therethrough in quantities offrom .1 to 2% by weight.

4. In the method of coating a substantially electrically non-conductivearticle made from material selected from the class consisting of rubber.and synthetic elastomers including a metal insert therein with asolvent containing paint, the step of heating the article to atemperature in the neighborhood of 200 to 205 F., passing the heatedarticle into an electrostatic field of defined character wherein thearticle itself forms. one. of the electrodes, and simultaneouslyspraying paint on the article wherein the particles of paint spray carrya charge opposite to that=carried by the article whereby a .uniform,strongly adhering coating .is obtained.

5. The methodas claimed in claim 4 including the added step .ofheating.the paint to a tempera ture of between 85 and 120 IT, prior to thespraying step.

6. The methodas claimed in claim 4 wherein the article includes from 1%to 2% by weight of a conductive carbon black intimately dispersedtherethrough.

7. In themethod of painting steeringwheels including a compounded rubbercovering on a reinforcing steel spider, the-steps .of; heating thesteering wheel to a temperature of approximately 200 F., passing saidsteering wheel in the heated condition into an electrostatic field of.defined character wherein the steering wheel forms one electrode of thecircuit, and simultaneously .spraying solvent containing paint at atempera-- ture of about F.,. onto the surface'of the wheel wherein theparticles of paint spray carry a charge Qpposite to that carried by thewheel for forming ,a uniform, strongly adhering .coating thereon, andfinally cooling the wheel so VANCE W. TOLLE. PAUL E. LUDY.

REFERENCES CITED The following references are of record in the file ofthis patent:

1. IN THE METHOD OF COATING A SUBSTANTIALLY ELECTRICALLY NON-CONDUCTIVEARTICLE MADE FROM A MATERIAL SELECTED FROM THE CLASS CONSISTING OFRUBBER AND SYNTHETIC ELASTOMERS, WHICH ARTICLE INCLUDES A METAL INSERTTHEREIN, WITH A SOLVENT CONTAINING PAINT, THE STEPS OF; HEATING THEARTICLE TO A TEMPERATURE OF FROM 150* F. TO 250* F., PASSING THE HEATEDARTICLE INTO AN ELECTROSTATIC FIELD OF DEFINED CHARACTER, ANDSIMULTANEOUSLY SPRAYING A SOLVENT CONTAINING PAINT ON THE ARTICLEWHEREBY A UNIFORM, STRONGLY ADHERING COATING IS OBTAINED.